Image processing device, image processing method, image creating device, computer software and record medium

ABSTRACT

A halftone reproduction processing section ( 29 ) of an image processing apparatus ( 13 ) has a control part that conducts a halftone processing of input image data by subjecting it to an error diffusion processing, selects input image data on at most one, namely, one or no input image data from the input image data on at least seven colors, namely, primary colors of cyan, magenta, and yellow used as visible colors for forming an image on a recording medium, secondary colors of red, green, and blue that are the complementary colors of the visible colors, and black, and subjects the input image data on the other colors to a different error diffusion processing.

TECHNICAL FIELD

This invention relates to an image processing apparatus, an imageforming apparatus, an image processing method, a computer softwareincluding an image processing program, and a recording medium having animage processing program recorded thereon, for reproducing an image byusing at least seven colors including primary colors of cyan, magenta,and yellow, secondary colors of red, green, and blue, and black, in acopier, printer, and the like.

BACKGROUND ART

In an inkjet recording device comprising a recording head for separatelydischarging ink for primary colors of cyan, magenta, and yellow andblack in response to a recording signal based on image data, there areproblems that (1) a secondary color of red, green, or blue is difficultto represent by adjusting primary colors and that (2) a good imagecannot be obtained because of severe color mixing at the boundarybetween different secondary colors. To alleviate these problems, thereis an inkjet recording device comprising a recording head for separatelydischarging ink for seven colors of cyan, magenta, yellow, black, red,green, and blue in response to a recording signal based on image data(see Patent Document 1).

According to the technique disclosed in Patent Document 1, ink per secan be adjusted without complex color processing, and the secondarycolors of red, green, and blue are recorded with ink of red, green, andblue, respectively. This can reduce ink injection quantity and preventthe occurrence of blur at the boundary between different colors, whichwould otherwise cause problems especially in secondary colors.

-   [Patent Document 1] JP 8-244254 A

DISCLOSURE OF INVENTION

When a halftoning process (e.g., multilevel error diffusion process) isapplied to color input image data to form an image on a recording mediumsuch as paper, an image having more excellent color reproducibility andvisual characteristics can be outputted by minimizing dot overlap andgenerating dots as uniformly as possible.

Patent Document 1 discloses a process of black generation and a processof secondary color generation from density signals of cyan, magenta, andyellow. However, depending on a subsequent method of generating data fordischarging ink, that is, a method of multilevel quantizationprocessing, there may be a pixel at which ink of a plurality of colorsexcessively overlaps, or conversely, a pixel at which the quantity ofink is excessively small. This is undesirable in view of colorreproducibility.

An object of the invention is to provide an image processing apparatus,an image forming apparatus, an image processing method, an imageprocessing program, and a recording medium having an image processingprogram recorded thereon, capable of outputting an image havingexcellent color reproducibility and visual characteristics withoutexcessively overlapping ink of seven colors of cyan, magenta, yellow,black, red, green, and blue for each pixel.

An image processing apparatus of the invention comprises a tonereproduction section for applying an error diffusion process to inputimage data to perform a halftoning process, wherein the tonereproduction section includes a control part that selects input imagedata of at most one, namely, not more than one color from the inputimage data of at least seven colors and that applies a different errordiffusion process to input image data of the other colors, the at leastseven colors including primary colors used as visible colors for formingan image on a recording medium, secondary colors that are complementarycolors of the visible colors, and black.

According to the image processing apparatus of the invention, outputimage data can be generated with avoiding dot overlap.

An image processing apparatus of the invention comprises a control partthat selects input image data having the largest density value among theinput image data of the seven colors.

According to the image processing apparatus of the invention, a dothaving the densest color among the seven colors can be outputted foreach pixel, and accumulation of errors can be minimized.

An image processing apparatus of the invention comprises a tonereproduction section for applying an error diffusion process to inputimage data to perform a halftoning process. The error diffusion processuses a threshold being set to not less than a half of the maximumdensity value that can be capable of the input image data when the tonereproduction section performs the error diffusion process on the inputimage data of at least seven colors including primary colors used asvisible colors for forming an image on a recording medium, secondarycolors that are complementary colors of the visible colors, and black.

According to the image processing apparatus of the invention, a processof comparing the image data of seven colors to determine a color havingthe maximum value can be eliminated by setting a threshold in order toselect at most one, namely, not more than one color for the dot to beoutputted among the image data of the seven colors for each pixel. Thenumber of colors outputted for the image data for the dot outputted bythe error diffusion process can thus be limited to not more than onecolor.

An image processing apparatus of the invention comprises a tonereproduction section for applying a multilevel error diffusion processto input image data to perform a halftoning process, wherein the tonereproduction section includes a control part for performing control soas to quantize the input image data of at least seven colors by themultilevel error diffusion process, to select input image data of atmost one color from the quantized values in accordance with a summationvalue of the input image data of the at least seven colors, and toforcedly adjust the selected input image data by one level to diffusequantization error, the at least seven colors including primary colorsused as visible colors for forming an image on a recording medium,secondary colors that are complementary colors of the visible colors,and black.

According to the image processing apparatus of the invention, quantizedvalues obtained by the multilevel error diffusion process are adjustedso that input image data of at most one color is selected from thequantized values in accordance with a summation value of the input imagedata of at least seven colors, and the selected input image data isforcedly adjusted by one level to diffuse quantization error. Thereforeimage data can be outputted with avoiding excessively large amount ofdot discharge and overlap. Moreover, image data can be outputted withavoiding excessively small amount of dot discharge.

An image processing apparatus of the invention is characterized in thatthe control part selects the color having the largest quantization errorand forcedly increases the quantized value of the selected color by onelevel when the summation value of the input image data of the at leastseven colors is larger than a value that is one level above the totalquantized values, and that the control part selects the color having thesmallest quantization error, that is, having the negatively largestabsolute value and forcedly decreases the quantized value of theselected color by one level when the summation value of the input imagedata of the at least seven colors is smaller than a value that is onelevel below the total quantized values.

According to the image processing apparatus of the invention, the colorhaving the largest quantization error is selected and the quantizedvalue of the selected color is forcedly increased by one level when thesummation value of the input image data of the at least seven colors islarger than a value that is one level above the total quantized values,and the color having the smallest quantization error, that is, havingthe negatively largest absolute value is selected and the quantizedvalue of the selected color is forcedly decreased by one level when thesummation value of the input image data of the at least seven colors issmaller than a value that is one level below the total quantized values.Therefore image data more suitable to the input data can be outputtedwhile avoiding excessively large amount of dot discharge and overlap.Moreover, image data more suitable to the input data can be outputtedwhile avoiding excessively small amount of dot discharge.

An image forming apparatus of the invention comprises theabove-described image processing apparatus.

According to the invention, excessively large amount of dot dischargeand overlap, and excessively small amount of dot discharge can beavoided. Therefore an image forming apparatus can be provided that canoutput an image having good quality.

An image processing method of the invention comprises a tonereproduction step of applying an error diffusion process to input imagedata to perform a halftoning process, wherein the tone reproduction stepselects input image data of at most one color from the input image dataof at least seven colors and applies a different error diffusion processto input image data of the other colors, the at least seven colorsincluding primary colors used as visible colors for forming an image ona recording medium, secondary colors that are complementary colors ofthe visible colors, and black.

According to the image processing method of the invention, output imagedata can be generated with avoiding dot overlap.

An image processing method of the invention comprises a tonereproduction step of applying an error diffusion process to input imagedata to perform a halftoning process, wherein the error diffusionprocess uses a threshold being set to not less than a half of themaximum density value that can be capable of the input image data whenthe tone reproduction section performs the error diffusion process onthe input image data of at least seven colors including primary colorsused as visible colors for forming an image on a recording medium,secondary colors that are complementary colors of the visible colors,and black.

According to the image processing method of the invention, a process ofcomparing the image data of seven colors to determine a color having themaximum value can be eliminated by setting a threshold in order toselect at most one color for the dot to be outputted among the imagedata of the seven colors for each pixel. The number of colors outputtedfor the image data for the dot outputted by the error diffusion processcan thus be limited to not more than one color.

An image processing method of the invention comprises a tonereproduction step of applying a multilevel error diffusion process toinput image data to perform a halftoning process. The tone reproductionstep is characterized by performing control so as to quantize the inputimage data of at least seven colors by the multilevel error diffusionprocess, to select input image data of at most one color from thequantized values in accordance with a summation value of the input imagedata of the at least seven colors, and to forcedly adjust the selectedinput image data by one level to diffuse quantization error, the atleast seven colors including primary colors used as visible colors forforming an image on a recording medium, secondary colors that arecomplementary colors of the visible colors, and black.

A computer software of the invention comprises a program that causes acomputer to perform the above-described image processing method.

According to the invention, when an image is formed using image data ofat least seven colors including primary colors of cyan, magenta, andyellow, secondary colors of red, green, and blue, and black, thecomputer can read and perform an image processing method of selecting atmost one color from the image data and forcedly adjusting the quantizedvalue by one level to make output image data in order to avoidexcessively large amount of dot discharge and overlap and excessivelysmall amount of dot discharge for each color. Hence the image processingmethod can be adapted to general purposes.

Moreover, when an image is formed using image data of at least sevencolors including primary colors of cyan, magenta, and yellow, secondarycolors of red, green, and blue, and black, the computer can read andperform an image processing method of selecting at most one, namely, notmore than one color from the image data to make output image data, or animage processing method of selecting at most one color from the imagedata and forcedly adjusting the quantized value by one level to makeoutput image data, in order to avoid dot overlap for each color. Hencethe image processing method can be adapted to general purposes.

A recording medium of the invention has the above-described imageprocessing method stored thereon so as to be executable by a computer.

According to the above configuration, a program of the image processingmethod can be easily supplied to a computer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of an imageprocessing apparatus provided in an image forming apparatus.

FIG. 2 is an illustration of the configuration of an inkjet recordingdevice.

FIG. 3 is a perspective view of a printhead.

FIG. 4 is an illustration of a halftone output gradation process.

FIG. 5 is an illustration of the configuration of a halftonereproduction processing section.

FIG. 6 is a flow chart of a halftoning process.

FIG. 7 is a block diagram of an error diffusion processing section.

FIG. 8 is a block diagram illustrating the configuration of a printerdriver provided in a computer.

FIG. 9 is an illustration of the configuration of a halftonereproduction processing section.

FIG. 10 is a flow chart of a halftoning process.

FIG. 11 is a block diagram of an error diffusion processing section.

BEST MODE FOR CARRYING OUT THE INVENTION FIRST EXAMPLE

FIG. 1 is a block diagram showing the configuration of an imageprocessing apparatus 13 provided in an image forming apparatus 11 of adigital copier or an inkjet copier (which may be a multifunction machineequipped with copier, facsimile, and printer functions).

The image processing apparatus 13 is implemented by an analog-to-digital(hereinafter abbreviated as “A/D”) conversion section 21, a shadingcorrecting section 22, an input tone correcting section 23, a colorcorrecting section 24, an segmentation processing section 25, a blackgeneration and under color removal section 26, a secondary colorgenerating section 27, a spatial filtering section 28, a halftonereproduction processing section (tone reproduction section) 29, and anoutput converting section 30. The A/D conversion section 21 converts RGB(R: red, G: green, B: blue) reflectance signals (reflectance data)provided from an image input device 12 into digital signals. The shadingcorrecting section 22 applies a shading correction process to the A/Dconverted reflectance signals. The shading correction process serves toremove various distortions that may occur in the image signals due tothe arrangement of illumination, image formation, and image pickupsystems of the image input device 12.

The input tone correcting section 23 applies an input gradationcorrection process to the shading corrected reflectance signals. Theinput gradation correction process is a process for converting thereflectance signals into signals being tractable for the imageprocessing apparatus 13 such as density signals. The input tonecorrecting section 23 may further apply a color balance process to thereflectance signals.

The color correcting section 24 converts the RGB (R: red, G: green, B:blue) density signals into CMY (C: cyan, M: magenta, Y: yellow) densitysignals and applies a color correction process to the CMY densitysignals for achieving fidelity of color reproduction in an image outputdevice 15. Specifically, the color correction process is a process forremoving, from the CMY density signals, color muddiness due to thespectral characteristics of CMY toner and ink containing unwantedabsorption components.

The segmentation processing section 25 performs an area separationprocess based on the CMY density signals outputted from the colorcorrecting section 24. The separated result (area identificationsignals) of the segmentation processing section 25 is provided to theblack generation and under color removal section 26 and the spatialfiltering section 28, and may be provided to the halftone reproductionprocessing section 29.

The black generation and under color removal section 26 performs a blackgeneration process for generating a black (K) color signal based on theCMY color signals constituting the density signals outputted from thecolor correcting section 24. Moreover, the black generation and undercolor removal section 26 applies an undercolor removing process to theCMY color signals. The undercolor removing process is a process forsubtracting the black color signal generated by the black generationprocess from the CMY color signals to obtain new CMY color signals. As aresult of these processes, the CMY density signals are converted into animage data, which is an image signal composed of CMYK color signals.

The secondary color generating section 27 performs a secondary colorgeneration process on two color signals among the three CMY colorsoutputted from the black generation and under color removal section 26.The secondary color generation process generates secondary colors fromthe two color signals among the three CMY colors and subtracts thesecondary color signals from the two colors. The generated secondarycolor (complementary color of a visible color) is blue for thecombination of cyan and magenta, green for the combination of cyan andyellow, or red for the combination of magenta and yellow, as describedlater in detail.

The spatial filtering section 28 applies a spatial filtering processusing a digital filter to the CMYKRGB image data obtained by the blackgeneration and under color removal section 26 and the secondary colorgenerating section 27. This corrects the spatial frequencycharacteristics of the image, and therefore any blur or granulardegradation can be avoided in the image outputted from the image outputdevice 14.

The halftone reproduction processing section (tone reproduction section)29 applies a gradation correction process and a halftoning process tothe spatial filtered CMYK image data. The halftoning process is aprocess for dividing the image into a plurality of pixels to reproducegradation and can use techniques such as two-level or multileveldithering and error diffusion. The half tone reproduction processingsection 29 may perform a process of converting the density value of theimage data into a halftone dot area rate, which is a characteristicvalue of the image output device 14.

The output converting section 30 converts the output image data inaccordance with an array of heads (C, M, Y, K, or other inkjet heads) ofthe inkjet recording device that serves as the image output device 14.The density signals processed by the output converting section 30 areprovided to the image output device 14. The foregoing operations arecontrolled by a CPU (Central Processing Unit, or control part), forexample.

FIG. 2 is a perspective view showing the overall configuration of aninkjet recording device (copier, printer, or multifunction machine),which is an example of the image output device.

The inkjet recording (image output) device 14 is generally composed ofan printhead (inkjet head) 141, a carriage 142, conveying rollers 143, aguide shaft 144, a holding means 145, a motor 146, a driving belt 147,and a maintenance unit 148. The carriage 142 has the printhead (inkjethead) 141 mounted thereon and can be moved in the main scan directionindicated by arrows X1 and X2 relative to a recording medium P. Therecording medium P is fed from a feeder, not shown, in the sub-scandirection indicated by arrow Y, whereas image formation is performed bythe printhead 141 moving in the direction of arrows X1 and X2. Therecording media stored in the feeder are fed out one sheet at a time bya feeding roller, not shown, and supplied to the printhead 141 sectionby the conveying rollers 143 serving as a recording medium conveyingmeans. The recording medium P that has finished recording is ejected toan ejector (not shown) The printhead (inkjet head) 141 is slidablysupported on the guide shaft 144 and the holding means 145 extending inthe main scan direction and its position relative to the recordingmedium is determined. Moreover, the driving belt 147 driven by the motor146 serving as a driving means is stretched parallel to the guide shaft144. The printhead (inkjet head) 141 is driven and displaced by thedriving belt 147. Note that in the maintenance unit 148, maintenancesuch as cleaning for the printhead (inkjet head) 141 is performed.

As shown in FIG. 3, the printhead (inkjet head) 141 comprises ink tanks151 for ink of a plurality of colors, for example, normal ink of sevencolors consisting of C (cyan), M (magenta), Y (yellow), K (black), R(red), G (green), and B (blue) These kinds of ink are discharged from anozzle 161 onto the recording medium in response to image data.

In the configuration of the image processing apparatus and imageprocessing method according to this embodiment, an error diffusionprocess is used to output two-level data indicating whether the halftoneoutput (reproduction) gradation processing section 29 discharges a dotof ink.

Reference is made to FIG. 4 to describe the processes from the blackgeneration and undercolor removing processes to the halftone outputgradation process in this embodiment. FIG. 4 illustrates an example ofthe processes from the black generation and undercolor removingprocesses to the secondary color generation process.

The black generation and under color removal section 26 performs blackgeneration and undercolor removing processes based on the CMY colorsignals constituting the density signals outputted from the colorcorrecting section 24.

In the black generation process according to this embodiment, theminimum of the CMY color signals (FIG. 4(a)) is used directly as a blackcolor signal, which is then subtracted from each of the CMY signals. Inthis way, at least one of the three CMY color density signals is set tozero. For example, if the cyan color signal is 200, the magenta colorsignal is 150, and the yellow color signal is 60, then as shown in FIG.4(b), their minimum, 60, is used as a black color signal, which issubtracted from each of the CMY color signals to obtain a cyan colorsignal of 140, a magenta color signal of 90, and a yellow signal of 0(FIG. 4(c)).

The secondary color generating section 27 performs a secondary colorgeneration process on nonzero color signals among the three CMY colorsoutputted from the black generation and under color removal section 26.In the secondary color generation process according to this embodiment,the minimum of the two nonzero color signals among the three CMY colorsignals is used as secondary color signals, which are subtracted fromthe two original color signals. In this way, at least one of the twocolors is set to zero. As shown in FIG. 4(d), given a cyan color signalof 140 and a magenta color signal of 90, their minimum, 90, is used as ablue signal, which is subtracted from the cyan and magenta color signalsto obtain a cyan color signal of 50 and a magenta color signal of 0(FIG. 4(e)).

The processes in the black generation and under color removal section 26and the secondary color generating section 27 result in at most threenonzero color signals, that is, one color among the three primarycolors, one color among the secondary colors, and black. After theseprocesses, the image data of seven CMYKRGB colors is subjected to aspatial filtering process in the spatial filtering section 28.

Moreover, the halftone reproduction processing section 29 performsgradation correction and halftoning processes.

The halftone reproduction processing section 29 comprises an output tonecorrecting section 41 and a halftoning section 42 (see FIG. 5).

The output tone correcting section 41 performs a process of converting,for each pixel, the CMYK image data into a halftone dot area rate, whichis a characteristic value dependent on the output characteristics of theimage output device 14.

The halftoning section 42 is now described in detail (see the blockdiagram of FIG. 7 for the error diffusing section 43).

The halftoning section 42 comprises error diffusing sections 43 and amaximum density determination section 44. Given a pixel of receivedinput image data, the error diffusing section 43 uses an adder 431 toadd to the pixel a diffusion error for the pixel stored in an errorstoring section 437, and outputs the result to a quantizing section 432.Based on a density determination signal from the maximum densitydetermination section 44, the quantizing section 432 selects aquantization threshold stored in a quantization threshold storingsection 433 and compares it with the pixel to which the diffusion errorhas been added, thereby performing binarization and determining an errorrelative to the quantized value. A diffusion coefficient setting section435 selects a diffusion coefficient from a diffusion coefficient storingsection 436 and calculates a diffusion error based on this diffusioncoefficient and the above-described error. The diffusion error is thenstored in the error storing section 437.

The maximum density determination section 44 determines the densest dataamong the CMYKRGB image data and outputs a signal (density determinationsignal) to the quantizing section 432 for the densest color, the signaldirecting that the densest color be quantized with two levels using anormal threshold and the associated error be diffused. For the othercolors, a signal (density determination signal) is outputted, directingthat the quantization threshold stored in the quantization thresholdstoring section 433 of the error diffusing section 43 be set to 255.Alternatively, a signal (density determination signal) is outputted tothe quantizing section 432, directing to unconditionally set thequantized value to zero. Here the threshold of 255 is the maximumdensity when the image is represented with 256 levels of gradation.

FIG. 6 illustrates a flow of the halftoning process in the halftoningsection 42 of the halftone reproduction processing section 29 accordingto this embodiment. It is assumed here that the image output deviceforms an image by switching dot discharge on and off, that is, by usinga two-level image, and a two-level error diffusion technique is used asthe halftoning process.

At step ST10, for each of the CMYK color components, the accumulatederror diffused to the processed pixel is added to the input image data.Then, at step ST11, a color component having the largest value isdetermined among the input image data of the addition result. At stepST12, the threshold for the color components other than the largest oneis set to 255 and each color component is compared with the threshold(i.e., 255) to perform binarization, thereby diffusing the binarizationerror (step ST13).

After the secondary color generation process in the secondary colorgenerating section 27, the nonzero image data has three colors, that is,one color among the three primary CMY colors, one color among thesecondary RGB colors, and black. Therefore, after the spatial filteringprocess, the image data of the color components other than these threecolors is also close to zero. It is thus acceptable to binarize one ofthese three color components having the largest image data (densityvalue) after the spatial filtering process and to perform errordiffusion based thereon. The image data for colors other than the colorof the largest image data is subjected to the error diffusion processwith the threshold being forcedly set to 255. Alternatively, withoutperforming the threshold determination process, the quantized value oroutput value may be unconditionally set to zero to diffuse thequantization error.

In view of 255 (the maximum density for the image represented with 256levels of gradation)/2=127.5, it is also acceptable to binarize eachcolor using 127 (dropping the fractional portion) as a threshold of eachcolor without examining which image data has the largest value. This isbecause two or more colors among the three colors are by no means 128 orhigher, since the total density value of the three colors is assumed tobe 255 at maximum. In this case, the maximum density determinationsection in FIGS. 5 and 7 is not needed.

According to the foregoing process, dots of a plurality of colors are byno means discharged on any one pixel. Therefore degradation ofreproducibility can be restrained and a visually desirable image can beobtained.

According to the invention, a program capable of performing this imageprocessing method may be generated and stored on a recording medium suchas a hard disk, flexible disk, or CD-ROM. The above-describedconfiguration enables an image forming apparatus or computer to beeasily supplied with the above recording medium and to perform thisimage processing method.

This program can be applied to a computer as a software, for example.Alternatively, a printer driver including the program may be installedon a computer.

FIG. 8 is a block diagram illustrating the configuration of a printerdriver provided in a computer. The computer 50 comprises a printerdriver 51, a communication port driver 56, and a communication port 57.The printer driver 51 comprises a color correcting section 52, ahalftone reproducing section 53, an output converting section 54, and aprinter language translating section 55. The computer 50 is connected toan inkjet printer (image output device) 14 via the communication portdriver 56 and the communication port 57 for RS232C, LAN, or the like.

In the computer 50, an image data generated by executing variousapplication programs is passed to the color correcting section 52 in theprinter driver 51. The color correcting section 52 applies a process tothe image data, the process being similar to that of the colorcorrecting section provided in the above-described color imageprocessing apparatus. The color correcting section 52 also performs thesame processes as those in the black generation and under color removalsection and the secondary color generating section of the color imageprocessing apparatus. The image data outputted from the color correctingsection 52 is then passed to the halftone reproducing section 53 wheregradation correction and halftoning processes are applied. After beingsubjected to the processes in the halftone reproducing section 53, theimage data is passed to the output converting section 54. The outputconverting section 54 converts the output image data in accordance withan array of heads (C, M, Y, K, or other inkjet heads) of the printer 14and passes it to the printer language translating section 55 where it isconverted into the printer language. The image data outputted from theprinter language translating section 55 is inputted to the printer 14via the communication port driver 56 and the communication port 57. Theprinter 14 outputs the received image data onto a recording medium suchas paper.

The program and recording medium according to the invention can beprovided as a printer driver as described above to be added to theexisting computer, printer, and the like. This enables to easilyimplement the image processing method of the invention.

SECOND EXAMPLE

The image processing apparatus and image processing method according tothis example is characterized by a halftone reproduction processingsection that outputs a multilevel data indicating ink dot dischargequantity using a multilevel error diffusion process.

In this example, a gradation correcting process converts, for eachpixel, the CMYK image data into a halftone dot area rate, which is acharacteristic value dependent on the output characteristics of theimage output device 14.

The halftone reproduction processing section comprises an output tonecorrecting section 410 and a halftoning section 420 (see FIG. 9).

The halftoning section 420 comprises error diffusing sections 430 and aquantization summation value range determination section 440.

Referring to the block diagram shown in FIG. 11, the configuration ofthe error diffusing section 430 is described.

Given a pixel of received input image data, the error diffusing sections430 uses an adder 510 to add to the pixel a diffusion error for thepixel stored in an error storing section 570, and outputs the result toa quantizing section 520. The quantizing section 520 selects a set ofthree quantization thresholds (described later) stored in a quantizationthreshold storing section 530 and compares it with the image data of thepixel to which the diffusion error has been added, thereby performingfour-level quantization and determining an error relative to thequantized value.

A quantized value correcting section (control part) 540 determines asummation value of the quantized values for the color components or asummation value of the output data indicating dot discharge quantity andcompares it with a summation value range determination signal. Thesummation value range determination signal is outputted from thequantization summation value range determination section 440 thatdetermines the range of the summation value of the output data asdescribed later.

When the summation value of the quantized values for the colorcomponents or the summation value of the output data indicating dotdischarge quantity is larger than the range value indicated by thesummation value range determination signal, the quantized valuecorrecting section (control part) forcedly decreases the quantized valueof the color component having the negatively largest absolute value ofthe quantization error by one level and redetermines the quantizationerror.

When the summation value of the quantized values for the colorcomponents or the summation value of the output data indicating dotdischarge quantity is smaller than the range value indicated by thesummation value range determination signal, the quantized valuecorrecting section (control part) 540 forcedly increases the quantizedvalue of the color component having the largest quantization error byone level and redetermines the quantization error.

When the summation value of the quantized values for the colorcomponents or the summation value of the output data indicating dotdischarge quantity matches the range indicated by the summation valuerange determination signal, no correction of the quantized values isperformed. This process will be described later in detail.

A diffusion coefficient setting section 550 selects a diffusioncoefficient from a diffusion coefficient storing section 560, calculatesa diffusion error based on this diffusion coefficient and theabove-described error, and stores the diffusion error in the errorstoring section 570.

In order to prevent excessively large amount of dot discharge andoverlap or excessively small amount of dot discharge for the densityindicated by the input image data, the quantization summation valuerange determination section 440 determines a summation value of theCMYKRGB image data, and in accordance with the summation value,determines the range of the summation value of the quantized values forthe color components or the summation value of the output dataindicating total dot discharge quantity. In this example, it is assumedthat the image output device can adjust the quantity of dot dischargefrom a nozzle 161 shown in FIG. 3 in four levels. In other words, theimage is formed with multiple levels and a four-level error diffusiontechnique is used as the halftoning process. Quantized values for imagedata 0, 1, 2, and 3 indicating dot discharge quantity are set to 0, 85,170, and 255, respectively.

The summation value range determination signal is outputted as a signaldirecting that the summation value of the quantized values for the colorcomponents be adjusted to 0 or 85 or the summation value of the imagedata indicating dot discharge quantity be adjusted to 0 or 1 when thesummation value of the CMYKRGB image data is not more than 85, thesummation value of the quantized values for the color components beadjusted to 85 or 170 or the summation value of the image dataindicating dot discharge quantity be adjusted to 1 or 2 when thesummation value of the CMYKRGB image data is not less than 86 and notmore than 170, and the summation value of the quantized values for thecolor components be adjusted to 170 or 255 or the summation value of theimage data indicating dot discharge quantity be adjusted to 2 or 3 whenthe summation value of the CMYKRGB image data is not less than 171.

Referring now to the flow chart shown in FIG. 10, a flow of thehalftoning process according to this embodiment is described.

For each color component inputted to the error diffusing section 430, adiffusion error stored in the error storing section 570 is added to eachprocessed pixel by the adder 510 (step ST1). A summation value of theinput image data is determined for the color component to which theaccumulated error has been added, and the range of the summationquantity of the quantized values (output dot quantities from the nozzle)is determined (step ST2) At step ST3, a set of three quantizationthresholds (described later) stored in the quantization thresholdstoring section 530 is selected, the image data of the pixel to whichthe diffusion error has been added is compared with the quantizationthresholds to determine quantized values, and an error between the imagedata and the quantized value is determined. At step ST4, a summationvalue of the quantized values (output dot quantities) for the colorcomponents is determined.

At step ST5, it is determined whether the summation value of thequantized values (output dot quantities) is larger than the summationvalue range determination signal. When the summation value of thequantized values for the color components or the summation value of theoutput data indicating dot discharge quantity is larger than the rangevalue indicated by the summation value range determination signal,control proceeds to step ST7, where the quantized value of the colorcomponent having the negatively largest absolute value of thequantization error is forcedly decreased by one level and thequantization error is redetermined.

When the summation value of the quantized values for the colorcomponents or the summation value of the output data indicating dotdischarge quantity is not larger than the range value indicated by thesummation value range determination signal, control proceeds to stepST6, where it is determined whether the summation value of the quantizedvalues (output dot quantities) is smaller than the summation value rangedetermination signal. When the summation value of the quantized valuesfor the color components or the summation value of the output dataindicating dot discharge quantity is smaller than the range valueindicated by the summation value range determination signal, controlproceeds to step ST8, where the quantized value of the color componenthaving the largest quantization error is forcedly increased by one leveland the quantization error is redetermined. When the summation value ofthe quantized values for the color components or the summation value ofthe output data indicating dot discharge quantity is not smaller thanthe range value indicated by the summation value range determinationsignal, control proceeds to step ST9, where the diffusion error for eachcolor component is stored in the error storing section 570.

Assuming here that the quantization thresholds stored in thequantization threshold storing section 530 are 42, 127, and 212, thequantization process and the subsequent correction of quantized valuesin response to the summation value range determination signal willproceed as follows.

In the image data of the pixel to which the diffusion error has beenadded, assume that the cyan color signal is 50, the magenta color signalis 0, the yellow signal is 0, the red color signal is 0, the green colorsignal is 0, the blue color signal is 90, and the black color signal is60. The summation value of the CMYKRGB image data is then 200, which isnot less than 171. Therefore the quantization summation value rangedetermination section 440 outputs a summation value range determinationsignal directing that the summation value of the quantized values forthe color components be 170 or 255, that is, the summation value of theimage data indicating dot discharge quantity be 2 or 3. After thequantization process is applied, the quantized value of cyan is 85 (dotdischarge quantity of 1), the quantized value of magenta is 0 (dotdischarge quantity of 0), the quantized value of yellow is 0 (dotdischarge quantity of 0), the quantized value of red is 0 (dot dischargequantity of 0), the quantized value of green is 0 (dot dischargequantity of 0), the quantized value of blue is 85 (dot dischargequantity of 1), and the quantized value of black is 85 (dot dischargequantity of 1). The summation value of the quantized values for thecolor components is 255 (the summation value of the image dataindicating dot discharge quantity is 3), which is within the summationvalue range indicated by the summation value range determination signalof the quantization summation value range determination section 440. Inthis case, therefore, no correction of the quantized values is performedfor any of the color components, and the quantization error is directlydiffused.

In the image data of the pixel to which the diffusion error has beenadded, assume that the cyan color signal is 135, the magenta colorsignal is 0, the yellow signal is 0, the red color signal is 0, thegreen color signal is 0, the blue color signal is 45, and the blackcolor signal is 60. The summation value of the CMYKRGB image data isthen 240, which is not less than 171. Therefore the quantizationsummation value range determination section 440 outputs a summationvalue range determination signal directing that the summation value ofthe quantized values for the color components be 170 or 255, that is,the summation value of the image data indicating dot discharge quantitybe 2 or 3. After the quantization process is applied, the quantizedvalue of cyan is 170 (dot discharge quantity of 2), the quantized valueof magenta is 0 (dot discharge quantity of 0), the quantized value ofyellow is 0 (dot discharge quantity of 0), the quantized value of red is0 (dot discharge quantity of 0), the quantized value of green is 0 (dotdischarge quantity of 0), the quantized value of blue is 85 (dotdischarge quantity of 1), and the quantized value of black is 85 (dotdischarge quantity of 1). The summation value of the quantized valuesfor the color components is 340 (the summation value of the image dataindicating dot discharge quantity is 4), which exceeds the summationvalue range of 2 or 3 indicated by the summation value rangedetermination signal of the quantization summation value rangedetermination section 440. In this case, it is examined which of thecolor components has the negatively largest absolute value of thequantization error. The quantization error of cyan is −35, thequantization error of magenta is 0, the quantization error of yellow is0, the quantization error of red is 0, the quantization error of greenis 0, the quantization error of blue is −40, and the quantization errorof black is −25. The quantization error of blue has the negativelylargest absolute value. Hence the quantized value of blue is decreasedby one level (the dot discharge quantity is decreased by one) to 0 (dotdischarge quantity of 0), which results in the quantization error ofblue being 45. Subsequently, the quantization error is diffused for eachcolor component.

In the image data of the pixel to which the diffusion error has beenadded, assume that the cyan color signal is 0, the magenta color signalis 30, the yellow signal is 5, the red color signal is 25, the greencolor signal is 0, the blue color signal is 0, and the black colorsignal is 40. The summation value of the CMYKRGB image data is then 100,which is not less than 86 and not more than 170. Therefore thequantization summation value range determination section 440 outputs asummation value range determination signal directing that the summationvalue of the quantized values for the color components be 85 or 170,that is, the summation value of the image data indicating dot dischargequantity be 1 or 2. After the quantization process is applied, thequantized value of cyan is 0 (dot discharge quantity of 0), thequantized value of magenta is 0 (dot discharge quantity of 0), thequantized value of yellow is 0 (dot discharge quantity of 0), thequantized value of red is 0 (dot discharge quantity of 0), the quantizedvalue of green is 0 (dot discharge quantity of 0), the quantized valueof blue is 0 (dot discharge quantity of 0), and the quantized value ofblack is 0 (dot discharge quantity of 0). The summation value of thequantized values for the color components is 0 (the summation value ofthe image data indicating dot discharge quantity is 0), which is smallerthan the summation value range of 1 or 2 indicated by the summationvalue range determination signal of the quantization summation valuerange determination section 440. In this case, it is examined which ofthe color components has the largest quantization error. Thequantization error of cyan is 0, the quantization error of magenta is30, the quantization error of yellow is 5, the quantization error of redis 25, the quantization error of green is 0, the quantization error ofblue is 0, and the quantization error of black is 40. The quantizationerror of black is the largest. Hence the quantized value of black isincreased by one level (the dot discharge quantity is increased by one)to 85 (dot discharge quantity of 1), which results in the quantizationerror of black being −45. Subsequently, the quantization error isdiffused for each color component.

According to the foregoing process, excessively large amount of dotdischarge and overlap can be eliminated, and conversely, excessivelysmall amount of dot discharge can be prevented. Therefore degradation ofreproducibility can be restrained and a visually desirable image can beobtained.

According to the invention, a program capable of performing this imageprocessing method may be generated and stored on a recording medium suchas a hard disk, FD, or CD-ROM. The above-described configuration enablesan image forming apparatus or computer to be easily supplied with theabove recording medium and to perform this image processing method.

This program can be applied to a computer as a software, for example.Alternatively, a printer driver including the program may be installedon a computer.

The printer driver provided in a computer is as described in the firstexample.

The program and recording medium according to the invention can beprovided as a printer driver as described above to be added to theexisting computer, printer, and the like. This enables to easilyimplement the image processing method of the invention.

While a digital color copier and a printer are illustrated as the imageoutput device in this embodiment, the image output device mayalternatively be a digital multifunction machine having printer, copier,and facsimile functions.

INDUSTRIAL APPLICABILITY

The invention enables to form an image having excellent colorreproducibility and visual characteristics without overlapping ink ofseven colors of cyan, magenta, yellow, black, red, green, and blue foreach pixel.

Moreover, the invention can prevent excessive dot overlapping, orconversely, dot missing, thereby restraining degradation of imagequality.

1. An image processing apparatus comprising a tone reproduction sectionfor applying an error diffusion process to input image data to perform ahalftoning process, wherein the tone reproduction section includes acontrol part that selects input image data of at most one color from theinput image data of at least seven colors and that applies a differenterror diffusion process to input image data of the other colors, the atleast seven colors including primary colors used as visible colors forforming an image on a recording medium, secondary colors that arecomplementary colors of the visible colors, and black.
 2. An imageprocessing apparatus according to claim 1, wherein the control partselects input image data having the largest density value among theinput image data of the seven colors.
 3. An image processing apparatuscomprising a tone reproduction section for applying an error diffusionprocess to input image data to perform a halftoning process, wherein theerror diffusion process uses a threshold being set to not less than ahalf of the maximum density value that can be capable of the input imagedata when the tone reproduction section performs the error diffusionprocess on the input image data of at least seven colors includingprimary colors used as visible colors for forming an image on arecording medium, secondary colors that are complementary colors of thevisible colors, and black.
 4. An image processing apparatus comprising atone reproduction section for applying a multilevel error diffusionprocess to input image data to perform a halftoning process, wherein thetone reproduction section includes a control part for performing controlso as to quantize the input image data of at least seven colors by themultilevel error diffusion process, to select input image data of atmost one color from the quantized values in accordance with a summationvalue of the input image data of the at least seven colors, and toforcedly adjust the selected input image data by one level to diffusequantization error, the at least seven colors including primary colorsused as visible colors for forming an image on a recording medium,secondary colors that are complementary colors of the visible colors,and black.
 5. An image processing apparatus according to claim 4,wherein the control part performs control so that the control partselects the color having the largest quantization error and forcedlyincreases the quantized value of the selected color by one level whenthe summation value of the input image data of the at least seven colorsis larger than a value that is one level above the total quantizedvalues, and that the control part selects the color having the smallestquantization error (having the negatively largest absolute value) andforcedly decreases the quantized value of the selected color by onelevel when the summation value of the input image data of the at leastseven colors is smaller than a value that is one level below the totalquantized values.
 6. An image forming apparatus comprising an imageprocessing apparatus according to claim
 1. 7. An image processing methodcomprising a tone reproduction step of applying an error diffusionprocess to input image data to perform a halftoning process, wherein thetone reproduction step selects input image data of at most one colorfrom the input image data of at least seven colors and applies adifferent error diffusion process to input image data of the othercolors, the at least seven colors including primary colors used asvisible colors for forming an image on a recording medium, secondarycolors that are complementary colors of the visible colors, and black.8. An image processing method comprising a tone reproduction step ofapplying an error diffusion process to input image data to perform ahalftoning process, wherein the error diffusion process uses a thresholdbeing set to not less than a half of the maximum density value that canbe capable of the input image data when the tone reproduction sectionperforms the error diffusion process on the input image data of at leastseven colors including primary colors used as visible colors for formingan image on a recording medium, secondary colors that are complementarycolors of the visible colors, and black.
 9. An image processing methodcomprising a tone reproduction step of applying a multilevel errordiffusion process to input image data to perform a halftoning process,wherein the tone reproduction step performs control so as to quantizethe input image data of at least seven colors by the multilevel errordiffusion process, to select input image data of at most one color fromthe quantized values in accordance with a summation value of the inputimage data of the at least seven colors, and to forcedly adjust theselected input image data by one level to diffuse quantization error,the at least seven colors including primary colors used as visiblecolors for forming an image on a recording medium, secondary colors thatare complementary colors of the visible colors, and black.
 10. Acomputer software comprising a program that causes a computer to performan image processing method according to claim
 7. 11. A recording mediumon which a program according to claim 10 is stored so as to beexecutable by a computer.